In previous studies, heat treatment was carried out at temperatures below 180°C by impregnating a dilute inorganic salt aqueous solution into cedar (Cryptomeria japonica D. Don) sapwood. As a result, water insolubilization of the decomposition products generated by heat treatment was suggested when NH4Cl was used. Therefore, we aimed to investigate the mechanism by which the decay resistance is improved by heat treatment using NH4Cl. The changes occurring in wood components as the decay resistance was improved were investigated. The following became clear as that result.The cold water soluble component ratio hardly increased and the absolute amount ratio of the acid insoluble component became higher as NH4Cl concentration increased, and the acid insolubilization of the pyrolysate was observed. In the heat treatment for 24 hours, there were linear relationships between the mass loss rates by the decay resistance and the heat treatment, and the acid insoluble component ratio and by the mass loss rate by the decay resistance, and the decay resistance was improved with the increase of the mass loss, that is with increase of and the acid insoluble component. The sample heat-treated with 2.5% NH4Cl at 150°C met the performance criterion for preservation specified in JIS K 1571. In the heat treatment for 72 hours, even a smaller mass loss rate and an acid insoluble component ratio compared with the heat treatment for 24 hours, the decay resistance was improved than that of the heat treatment for 24 hours. When heat treatment was carried out at a concentration of 1.0% and 170°C for 72 hours, the mass loss rate with heat treatment was 11.3% and the sample was satisfied the performance criterion for decay resistance.By using NH4Cl, the performance criterion for decay resistance was achieved by heat treatment at temperatures greatly lower and with decreasing mass loss by more than 7% than conventional high-temperature treatment conducted above 200°C.
